1. Field of the Invention
The present invention relates to a foreign substance removing apparatus, and more particular, to a foreign substance removing apparatus for removing foreign substances adhered to a surface of a plate-like light transmitting member disposed in a light ray incident path.
The foreign substance removing apparatus is mounted to an original reading apparatus of a copying machine, a facsimile machine, a document scanner or the like or mounted to an image pickup apparatus such as a video camera and a still camera.
2. Description of the Related Art
In a recent image pickup apparatus, a shot image is much affected by dusts adhered to an optical system of the image pickup apparatus, with improvement of resolution of an optical sensor of the apparatus. In particular, with remarkable improvement of resolution of an image pickup device of a video camera, a still camera or the like, dusts are sometimes captured in a shot image. More specifically, when dusts from the outside or abrasive dusts produced by friction in the image pickup apparatus are adhered to an infrared ray cut filter, an optical low pass filter or the like disposed near the image pickup device, such dusts are captured in a shot image.
When reading a planar original disposed close to a line sensor of an image pickup unit of a facsimile machine, a document scanner or the like, the image pickup unit repetitively scans the original in a lengthwise direction of the line sensor (main scanning direction) while relatively moving the line sensor and the original in a sub-scanning direction. At this time, if dusts are adhered to a light ray incident part of the line sensor, the dusts are captured in the read image. In particular when dusts are adhered to the line sensor, the dusts are each captured in the image as a continuous line image extending in the sub-scanning direction, thus greatly impairing the quality of the image.
Although the desired image quality can be restored by a human intervention wiping foreign substances with a cloth or the like, a human operator is not aware of dust adherence until completion of shooting or image reading. When an image obtained by the shooting or image reading is in an electronic data form, the obtained image in which dust images have been captured can be corrected using an image processing software. However, laborious correcting operations are required. When the obtained image is output onto a paper medium, the paper medium is wastefully consumed.
Therefore, a camera with a dustproof mechanism that removes dusts using vibration has been proposed as disclosed in Japanese Laid-open Patent Publication Nos. 2002-204379 and 2003-333391, and an image reading apparatus using vibration to cause dusts to move away from an image reading unit has also been proposed as disclosed in Japanese Laid-open Publication Nos. 2003-280110 and 2004-012474.
FIG. 8 is a perspective view of an image reading apparatus disclosed in Japanese Laid-open Publication No. 2003-280110.
Referring to FIG. 8, a reader section 200 is provided that optically reads image information recorded on an original and optoelectrically converts the image information into image data. The reader section 200 includes a platen glass 201 for auto document feeder (ADF), hereinafter referred to as the “ADF platen 201”, a platen glass 202 for book-original, and a scanner unit 209 including a lamp 203 and a mirror 204. The reader section 200 further includes mirrors 205, 206, a lens 207, a CCD sensor (not shown), and the like.
When reading an image of an original conveyed from an ADF, not shown, the reader section 200 causes the scanner unit 209 to move to and stop at a location beneath the ADF platen 201, and reads image information while the original is conveyed along the ADF platen 201.
Reference numeral 210 denotes a piezoelectric element attached to a lower surface of the ADF platen 201. A high-frequency signal is applied from a circuit, not shown, to the piezoelectric element. As a result, bending vibration is produced in the ADF platen 201. The bending vibration removes foreign substances from the ADF platen 201, if they have been adhered to the ADF platen 201, whereby the foreign substances are prevented from being captured in a shot image. The piezoelectric element 210 is positioned so as not to hinder an optical image of an original from passing through the ADF platen 201 and reaching the scanner unit 209.
The ADF platen 201 is supported by a housing of the reader section 200 via supporting members made of an elastic material and disposed near the ADF platen 201. A foreign substance removing apparatus of the reader section 200 is comprised of the ADF platen 201, the piezoelectric element 210, and the supporting members.
FIGS. 9A through 9D are views showing bending vibrations produced in the ADF platen 201 by the piezoelectric element 210. The bending vibrations include multiple-order standing waves. FIG. 9B shows a first vibration mode, and FIG. 9C shows a second vibration mode. The first vibration mode is a seventh-order mode where eight nodes are present in the platen, and the second vibration mode is a sixth-order mode where seven nodes are present therein. As shown in FIG. 9D, each of the first and second vibration modes has a Y-directional vibration displacement distribution which is uniform without any nodes.
Vibrations in at least first and second vibration modes are sequentially applied from the piezoelectric element 210 to the ADF platen 201. As a result, vibrations are produced at various parts of the ADF platen 201. Since positions of the nodes in the first vibration mode are different from those in the second vibration mode, vibrations are produced at every position on the ADF platen 201. As a result, foreign substances are peeled off and moved away from the ADF platen 201.
However, the conventional foreign substance removing apparatus provided in the image reading apparatus in FIG. 8 poses the following problems.
FIG. 10A is a section view of the ADF platen 201 of the conventional foreign substance removing apparatus in FIG. 8 taken along an X-Z plane at a location near the piezoelectric element 210, and FIG. 11A is a section view of the ADF platen 201 taken along an X-Z plane at a location away from the piezoelectric element 210.
The ADF platen 201, which is a rectangular plate-like light transmitting member, is affixed with the circular plate-like piezoelectric element 210 using adhesive. FIG. 10A shows a sectional shape of the piezoelectric element 210 observed when a bending vibration in the piezoelectric element 210 causes a maximum Z-directional (thickness-directional) displacement of the piezoelectric element 210 at a location near the center 216 of the circular plate that forms the piezoelectric element 210. In FIGS. 10A and 11A, reference numeral 215 denotes a bending neutral surface, which represents a surface position where no X-directional elongation/contraction is produced by the bending vibration. In FIGS. 10B and 11B, the magnitudes of X-directional strains in the ADF platen 201 and the piezoelectric element 210 are represented by the lengths of arrows shown therein, in which the right pointing arrows represent elongation and the left pointing arrows represent contraction.
As shown in FIG. 11A, the bending neutral surface 215 is located at the thickness-directional (Z-directional) center of the ADF platen 201 in a part of the ADF platen 201 to which the piezoelectric element 210 is not affixed. On the other hand, in another part of the ADF platen 201 to which the piezoelectric element 210 is affixed and the rigidity of the piezoelectric element 210 is added, the bending neutral surface 215 is shifted toward the piezoelectric element 210 so as to be sometimes positioned in the piezoelectric element 210, as shown in FIG. 10A.
For the above reasons, the following two problems are caused.
First, an allowable vibration amplitude corresponding to a rupture limit of the ADF platen 201 decreases.
The ADF platen 201 made of an optical material such as glass or quartz is relatively low in rupture limit. When the bending neutral surface 215 is inside the ADF platen 201 as shown in FIG. 11A, the ratio of tensile stress in the upper and lower surfaces of the ADF platen 210 to amplitude of the bending vibration decreases to a minimum. On the other hand, at that part of the ADF platen 201 to which the piezoelectric element 210 is affixed, the bending neutral surface 215 is away from the ADF platen 201 as shown in FIG. 10A, and the tensile stress in the ADF platen 201 shown in FIG. 10A increases since the amount of strain varies in proportion to the distance from the neutral surface 215. As a result, the allowable vibration amplitude at which the rupture limit of the ADF platen 201 shown in FIG. 10A is reached decreases to be smaller than the allowable vibration amplitude corresponding to the rupture limit of the ADF platen 201 shown in FIG. 11A. Therefore, there is a risk that vibration having a sufficient amplitude cannot be applied to the ADF platen 201.
Second, an applicable force which the piezoelectric element 210 can apply to the ADF platen 201 decreases.
The piezoelectric element 210 is polarized in advance in the thickness direction (Z direction). A rear surface electrode is provided in a rear surface of the piezoelectric element 210 at which the piezoelectric element is affixed to the ADF platen 201, and a front surface electrode is provided in a front surface of the piezoelectric element, which is opposite from the rear surface. When a potential difference is applied across these electrodes, X-directional elongation/contraction stress is produced in the piezoelectric element 210 due to lateral electostrictive effect. In the piezoelectric element 210 which is in a state shown in FIG. 10A, a uniform contraction stress is produced in the piezoelectric element 210 in the thickness direction.
By virtue of the stress being produced, an X-directional elongation strain is produced in an upper region above the bending neutral surface 215 inside the piezoelectric element 210, and an X-directional contraction strain is produced in a lower region below the bending neutral surface 215.
As a result, despite that contraction stress is produced in a region 217 extending from the surface at which the piezoelectric element 210 and the ADF platen 201 are affixed together to the bending neutral surface 215, an elongation strain is produced in the region 217, and therefore, the stress does not effectively act. Specifically, in the region 217, even when a sufficient voltage is applied to the piezoelectric element 210, the desired vibration amplitude cannot be produced in the ADF platen 201.
An object of the present invention is to provide a foreign substance removing apparatus that can provide the desired vibration amplitude to a light transmitting member, without using a large driving voltage.